Continuity mistake: In the scene where one of the hall monitors is folding the principal's note into a paper airplane, watch his cigarette. It goes from being in his mouth, to being in his hand and back without him moving it.
Character mistake: When Tom is talking to Riff on the phone, he mentions that he is listening to the Ramones's latest album. However, the song playing as he says this is "I Wanna Be Your Boyfriend," which is from the Ramones's first album
Other mistake: Before the hall monitor folds the note into a paper airplane, the note has creases as is it was already folded into a paper airplane, and then flattened out for a different take.
Visible crew/equipment: After Miss Togar leaves the gym Coach tells the girls to hit the shower then blows her whistle, and when Riff tells the girls to go the taped T-mark can be seen on the floor at the bottom of the screen, just as the girls rush off (it is not a basketball line marking). (00:26:45)
Other mistake: While the school is burning, a few shots after the closeup of Kate and Tom kissing, there's another shot of an explosion and it's a flipped shot, note the writing is backwards on the banner. (01:30:10)
Continuity mistake: When Eaglebauer brings Kate to be Tom's "training partner" the rearview mirror in Eaglebauer's car vanishes, reappears, and vanishes again. (00:31:50)
Visible crew/equipment: While Eaglebauer and Kate are in the car, the reflection of the camera and lighting equipment can be seen on Eaglebauer's sunglasses. (00:31:50)
Other mistake: In the scene where The Ramones are playing "Pinhead", they say: "D-U-M-B" as the letters appear in a very big size. But when they say it again, it appears like this: "D-M-U-B." (01:08:30)
Continuity mistake: When the paper airplane flies into the classroom in the wideshot, there's a guy wearing a striped shirt sitting at the back who vanishes in the next closeup. (00:16:10)
Revealing mistake: After Fritz throws the paper airplane, when it flies into the first locker and comes out through the last locker, the wire pulling the paper can be seen here and then again in a following shot. (00:16:05)